Heat oven to 375°F.
Melt butter over medium heat in saucepan. Whisk in flour and cook a minute, then whisk in milk and season with salt, pepper and nutmeg. Simmer at a low bubble until thickened to coat spoon, stir in Dijon.
Arrange sliced bread on baking sheet and top with sauce, ham and cheese, bake to deep golden on top. Sandwiches can be made single or double layer, finishing top layer with sauce and cheese: bread, sauce, ham cheese, bread, sauce, cheese.